// Copyright 2013 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/base/platform/time.h"

#if V8_OS_POSIX
#include <fcntl.h> // for O_RDONLY
#include <sys/time.h>
#include <unistd.h>
#endif
#if V8_OS_MACOSX
#include <mach/mach.h>
#include <mach/mach_time.h>
#include <pthread.h>
#endif

#include <cstring>
#include <ostream>

#if V8_OS_WIN
#include "src/base/lazy-instance.h"
#include "src/base/win32-headers.h"
#endif
#include "src/base/cpu.h"
#include "src/base/logging.h"
#include "src/base/platform/platform.h"
//#include "patch_code/api_xp.h" // SUPPORT_XP_CODE

namespace {

#if V8_OS_MACOSX
int64_t ComputeThreadTicks()
{
    mach_msg_type_number_t thread_info_count = THREAD_BASIC_INFO_COUNT;
    thread_basic_info_data_t thread_info_data;
    kern_return_t kr = thread_info(
        pthread_mach_thread_np(pthread_self()),
        THREAD_BASIC_INFO,
        reinterpret_cast<thread_info_t>(&thread_info_data),
        &thread_info_count);
    CHECK_EQ(kr, KERN_SUCCESS);

    // We can add the seconds into a {int64_t} without overflow.
    CHECK_LE(thread_info_data.user_time.seconds,
        std::numeric_limits<int64_t>::max() - thread_info_data.system_time.seconds);
    int64_t seconds = thread_info_data.user_time.seconds + thread_info_data.system_time.seconds;
    // Multiplying the seconds by {kMicrosecondsPerSecond}, and adding something
    // in [0, 2 * kMicrosecondsPerSecond) must result in a valid {int64_t}.
    static constexpr int64_t kSecondsLimit = (std::numeric_limits<int64_t>::max() / v8::base::Time::kMicrosecondsPerSecond) - 2;
    CHECK_GT(kSecondsLimit, seconds);
    int64_t micros = seconds * v8::base::Time::kMicrosecondsPerSecond;
    micros += (thread_info_data.user_time.microseconds + thread_info_data.system_time.microseconds);
    return micros;
}
#elif V8_OS_POSIX
// Helper function to get results from clock_gettime() and convert to a
// microsecond timebase. Minimum requirement is MONOTONIC_CLOCK to be supported
// on the system. FreeBSD 6 has CLOCK_MONOTONIC but defines
// _POSIX_MONOTONIC_CLOCK to -1.
V8_INLINE int64_t ClockNow(clockid_t clk_id)
{
#if (defined(_POSIX_MONOTONIC_CLOCK) && _POSIX_MONOTONIC_CLOCK >= 0) || defined(V8_OS_BSD) || defined(V8_OS_ANDROID)
// On AIX clock_gettime for CLOCK_THREAD_CPUTIME_ID outputs time with
// resolution of 10ms. thread_cputime API provides the time in ns
#if defined(V8_OS_AIX)
    thread_cputime_t tc;
    if (clk_id == CLOCK_THREAD_CPUTIME_ID) {
        if (thread_cputime(-1, &tc) != 0) {
            UNREACHABLE();
        }
    }
#endif
    struct timespec ts;
    if (clock_gettime(clk_id, &ts) != 0) {
        UNREACHABLE();
    }
    // Multiplying the seconds by {kMicrosecondsPerSecond}, and adding something
    // in [0, kMicrosecondsPerSecond) must result in a valid {int64_t}.
    static constexpr int64_t kSecondsLimit = (std::numeric_limits<int64_t>::max() / v8::base::Time::kMicrosecondsPerSecond) - 1;
    CHECK_GT(kSecondsLimit, ts.tv_sec);
    int64_t result = int64_t { ts.tv_sec } * v8::base::Time::kMicrosecondsPerSecond;
#if defined(V8_OS_AIX)
    if (clk_id == CLOCK_THREAD_CPUTIME_ID) {
        result += (tc.stime / v8::base::Time::kNanosecondsPerMicrosecond);
    } else {
        result += (ts.tv_nsec / v8::base::Time::kNanosecondsPerMicrosecond);
    }
#else
    result += (ts.tv_nsec / v8::base::Time::kNanosecondsPerMicrosecond);
#endif
    return result;
#else // Monotonic clock not supported.
    return 0;
#endif
}

V8_INLINE bool IsHighResolutionTimer(clockid_t clk_id)
{
    // Limit duration of timer resolution measurement to 100 ms. If we cannot
    // measure timer resoltuion within this time, we assume a low resolution
    // timer.
    int64_t end = ClockNow(clk_id) + 100 * v8::base::Time::kMicrosecondsPerMillisecond;
    int64_t start, delta;
    do {
        start = ClockNow(clk_id);
        // Loop until we can detect that the clock has changed. Non-HighRes timers
        // will increment in chunks, i.e. 15ms. By spinning until we see a clock
        // change, we detect the minimum time between measurements.
        do {
            delta = ClockNow(clk_id) - start;
        } while (delta == 0);
    } while (delta > 1 && start < end);
    return delta <= 1;
}

#elif V8_OS_WIN
V8_INLINE bool IsQPCReliable()
{
    v8::base::CPU cpu;
    // On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is unreliable.
    return strcmp(cpu.vendor(), "AuthenticAMD") == 0 && cpu.family() == 15;
}

// Returns the current value of the performance counter.
V8_INLINE uint64_t QPCNowRaw()
{
    LARGE_INTEGER perf_counter_now = {};
    // According to the MSDN documentation for QueryPerformanceCounter(), this
    // will never fail on systems that run XP or later.
    // https://msdn.microsoft.com/library/windows/desktop/ms644904.aspx
    BOOL result = ::QueryPerformanceCounter(&perf_counter_now);
    DCHECK(result);
    USE(result);
    return perf_counter_now.QuadPart;
}
#endif // V8_OS_MACOSX

} // namespace

namespace v8 {
namespace base {

    int TimeDelta::InDays() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int>::max();
        }
        return static_cast<int>(delta_ / Time::kMicrosecondsPerDay);
    }

    int TimeDelta::InHours() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int>::max();
        }
        return static_cast<int>(delta_ / Time::kMicrosecondsPerHour);
    }

    int TimeDelta::InMinutes() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int>::max();
        }
        return static_cast<int>(delta_ / Time::kMicrosecondsPerMinute);
    }

    double TimeDelta::InSecondsF() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<double>::infinity();
        }
        return static_cast<double>(delta_) / Time::kMicrosecondsPerSecond;
    }

    int64_t TimeDelta::InSeconds() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int64_t>::max();
        }
        return delta_ / Time::kMicrosecondsPerSecond;
    }

    double TimeDelta::InMillisecondsF() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<double>::infinity();
        }
        return static_cast<double>(delta_) / Time::kMicrosecondsPerMillisecond;
    }

    int64_t TimeDelta::InMilliseconds() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int64_t>::max();
        }
        return delta_ / Time::kMicrosecondsPerMillisecond;
    }

    int64_t TimeDelta::InMillisecondsRoundedUp() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int64_t>::max();
        }
        return (delta_ + Time::kMicrosecondsPerMillisecond - 1) / Time::kMicrosecondsPerMillisecond;
    }

    int64_t TimeDelta::InMicroseconds() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int64_t>::max();
        }
        return delta_;
    }

    int64_t TimeDelta::InNanoseconds() const
    {
        if (IsMax()) {
            // Preserve max to prevent overflow.
            return std::numeric_limits<int64_t>::max();
        }
        return delta_ * Time::kNanosecondsPerMicrosecond;
    }

#if V8_OS_MACOSX

    TimeDelta TimeDelta::FromMachTimespec(struct mach_timespec ts)
    {
        DCHECK_GE(ts.tv_nsec, 0);
        DCHECK_LT(ts.tv_nsec,
            static_cast<long>(Time::kNanosecondsPerSecond)); // NOLINT
        return TimeDelta(ts.tv_sec * Time::kMicrosecondsPerSecond + ts.tv_nsec / Time::kNanosecondsPerMicrosecond);
    }

    struct mach_timespec TimeDelta::ToMachTimespec() const
    {
        struct mach_timespec ts;
        DCHECK_GE(delta_, 0);
        ts.tv_sec = static_cast<unsigned>(delta_ / Time::kMicrosecondsPerSecond);
        ts.tv_nsec = (delta_ % Time::kMicrosecondsPerSecond) * Time::kNanosecondsPerMicrosecond;
        return ts;
    }

#endif // V8_OS_MACOSX

#if V8_OS_POSIX

    TimeDelta TimeDelta::FromTimespec(struct timespec ts)
    {
        DCHECK_GE(ts.tv_nsec, 0);
        DCHECK_LT(ts.tv_nsec,
            static_cast<long>(Time::kNanosecondsPerSecond)); // NOLINT
        return TimeDelta(ts.tv_sec * Time::kMicrosecondsPerSecond + ts.tv_nsec / Time::kNanosecondsPerMicrosecond);
    }

    struct timespec TimeDelta::ToTimespec() const
    {
        struct timespec ts;
        ts.tv_sec = static_cast<time_t>(delta_ / Time::kMicrosecondsPerSecond);
        ts.tv_nsec = (delta_ % Time::kMicrosecondsPerSecond) * Time::kNanosecondsPerMicrosecond;
        return ts;
    }

#endif // V8_OS_POSIX

#if V8_OS_WIN

    // We implement time using the high-resolution timers so that we can get
    // timeouts which are smaller than 10-15ms. To avoid any drift, we
    // periodically resync the internal clock to the system clock.
    class Clock final {
    public:
        Clock()
            : initial_ticks_(GetSystemTicks())
            , initial_time_(GetSystemTime())
        {
        }

        Time Now()
        {
            // Time between resampling the un-granular clock for this API (1 minute).
            const TimeDelta kMaxElapsedTime = TimeDelta::FromMinutes(1);

            MutexGuard lock_guard(&mutex_);

            // Determine current time and ticks.
            TimeTicks ticks = GetSystemTicks();
            Time time = GetSystemTime();

            // Check if we need to synchronize with the system clock due to a backwards
            // time change or the amount of time elapsed.
            TimeDelta elapsed = ticks - initial_ticks_;
            if (time < initial_time_ || elapsed > kMaxElapsedTime) {
                initial_ticks_ = ticks;
                initial_time_ = time;
                return time;
            }

            return initial_time_ + elapsed;
        }

        Time NowFromSystemTime()
        {
            MutexGuard lock_guard(&mutex_);
            initial_ticks_ = GetSystemTicks();
            initial_time_ = GetSystemTime();
            return initial_time_;
        }

    private:
        static TimeTicks GetSystemTicks()
        {
            return TimeTicks::Now();
        }

        static Time GetSystemTime()
        {
            FILETIME ft;
            ::GetSystemTimeAsFileTime(&ft);
            return Time::FromFiletime(ft);
        }

        TimeTicks initial_ticks_;
        Time initial_time_;
        Mutex mutex_;
    };

    namespace {
        DEFINE_LAZY_LEAKY_OBJECT_GETTER(Clock, GetClock)
    }

    Time Time::Now() { return GetClock()->Now(); }

    Time Time::NowFromSystemTime() { return GetClock()->NowFromSystemTime(); }

    // Time between windows epoch and standard epoch.
    static const int64_t kTimeToEpochInMicroseconds = int64_t { 11644473600000000 };

    Time Time::FromFiletime(FILETIME ft)
    {
        if (ft.dwLowDateTime == 0 && ft.dwHighDateTime == 0) {
            return Time();
        }
        if (ft.dwLowDateTime == std::numeric_limits<DWORD>::max() && ft.dwHighDateTime == std::numeric_limits<DWORD>::max()) {
            return Max();
        }
        int64_t us = (static_cast<uint64_t>(ft.dwLowDateTime) + (static_cast<uint64_t>(ft.dwHighDateTime) << 32)) / 10;
        return Time(us - kTimeToEpochInMicroseconds);
    }

    FILETIME Time::ToFiletime() const
    {
        DCHECK_GE(us_, 0);
        FILETIME ft;
        if (IsNull()) {
            ft.dwLowDateTime = 0;
            ft.dwHighDateTime = 0;
            return ft;
        }
        if (IsMax()) {
            ft.dwLowDateTime = std::numeric_limits<DWORD>::max();
            ft.dwHighDateTime = std::numeric_limits<DWORD>::max();
            return ft;
        }
        uint64_t us = static_cast<uint64_t>(us_ + kTimeToEpochInMicroseconds) * 10;
        ft.dwLowDateTime = static_cast<DWORD>(us);
        ft.dwHighDateTime = static_cast<DWORD>(us >> 32);
        return ft;
    }

#elif V8_OS_POSIX

    Time Time::Now()
    {
        struct timeval tv;
        int result = gettimeofday(&tv, nullptr);
        DCHECK_EQ(0, result);
        USE(result);
        return FromTimeval(tv);
    }

    Time Time::NowFromSystemTime()
    {
        return Now();
    }

    Time Time::FromTimespec(struct timespec ts)
    {
        DCHECK_GE(ts.tv_nsec, 0);
        DCHECK_LT(ts.tv_nsec, kNanosecondsPerSecond);
        if (ts.tv_nsec == 0 && ts.tv_sec == 0) {
            return Time();
        }
        if (ts.tv_nsec == static_cast<long>(kNanosecondsPerSecond - 1) && // NOLINT
            ts.tv_sec == std::numeric_limits<time_t>::max()) {
            return Max();
        }
        return Time(ts.tv_sec * kMicrosecondsPerSecond + ts.tv_nsec / kNanosecondsPerMicrosecond);
    }

    struct timespec Time::ToTimespec() const
    {
        struct timespec ts;
        if (IsNull()) {
            ts.tv_sec = 0;
            ts.tv_nsec = 0;
            return ts;
        }
        if (IsMax()) {
            ts.tv_sec = std::numeric_limits<time_t>::max();
            ts.tv_nsec = static_cast<long>(kNanosecondsPerSecond - 1); // NOLINT
            return ts;
        }
        ts.tv_sec = static_cast<time_t>(us_ / kMicrosecondsPerSecond);
        ts.tv_nsec = (us_ % kMicrosecondsPerSecond) * kNanosecondsPerMicrosecond;
        return ts;
    }

    Time Time::FromTimeval(struct timeval tv)
    {
        DCHECK_GE(tv.tv_usec, 0);
        DCHECK(tv.tv_usec < static_cast<suseconds_t>(kMicrosecondsPerSecond));
        if (tv.tv_usec == 0 && tv.tv_sec == 0) {
            return Time();
        }
        if (tv.tv_usec == static_cast<suseconds_t>(kMicrosecondsPerSecond - 1) && tv.tv_sec == std::numeric_limits<time_t>::max()) {
            return Max();
        }
        return Time(tv.tv_sec * kMicrosecondsPerSecond + tv.tv_usec);
    }

    struct timeval Time::ToTimeval() const
    {
        struct timeval tv;
        if (IsNull()) {
            tv.tv_sec = 0;
            tv.tv_usec = 0;
            return tv;
        }
        if (IsMax()) {
            tv.tv_sec = std::numeric_limits<time_t>::max();
            tv.tv_usec = static_cast<suseconds_t>(kMicrosecondsPerSecond - 1);
            return tv;
        }
        tv.tv_sec = static_cast<time_t>(us_ / kMicrosecondsPerSecond);
        tv.tv_usec = us_ % kMicrosecondsPerSecond;
        return tv;
    }

#endif // V8_OS_WIN

    // static
    TimeTicks TimeTicks::HighResolutionNow()
    {
        // a DCHECK of TimeTicks::IsHighResolution() was removed from here
        // as it turns out this path is used in the wild for logs and counters.
        //
        // TODO(hpayer) We may eventually want to split TimedHistograms based
        // on low resolution clocks to avoid polluting metrics
        return TimeTicks::Now();
    }

    Time Time::FromJsTime(double ms_since_epoch)
    {
        // The epoch is a valid time, so this constructor doesn't interpret
        // 0 as the null time.
        if (ms_since_epoch == std::numeric_limits<double>::max()) {
            return Max();
        }
        return Time(
            static_cast<int64_t>(ms_since_epoch * kMicrosecondsPerMillisecond));
    }

    double Time::ToJsTime() const
    {
        if (IsNull()) {
            // Preserve 0 so the invalid result doesn't depend on the platform.
            return 0;
        }
        if (IsMax()) {
            // Preserve max without offset to prevent overflow.
            return std::numeric_limits<double>::max();
        }
        return static_cast<double>(us_) / kMicrosecondsPerMillisecond;
    }

    std::ostream& operator<<(std::ostream& os, const Time& time)
    {
        return os << time.ToJsTime();
    }

#if V8_OS_WIN

    namespace {

        // We define a wrapper to adapt between the __stdcall and __cdecl call of the
        // mock function, and to avoid a static constructor.  Assigning an import to a
        // function pointer directly would require setup code to fetch from the IAT.
        DWORD timeGetTimeWrapper() { return timeGetTime(); }

        DWORD (*g_tick_function)
        (void) = &timeGetTimeWrapper;

        // A structure holding the most significant bits of "last seen" and a
        // "rollover" counter.
        union LastTimeAndRolloversState {
            // The state as a single 32-bit opaque value.
            int32_t as_opaque_32;

            // The state as usable values.
            struct {
                // The top 8-bits of the "last" time. This is enough to check for rollovers
                // and the small bit-size means fewer CompareAndSwap operations to store
                // changes in state, which in turn makes for fewer retries.
                uint8_t last_8;
                // A count of the number of detected rollovers. Using this as bits 47-32
                // of the upper half of a 64-bit value results in a 48-bit tick counter.
                // This extends the total rollover period from about 49 days to about 8800
                // years while still allowing it to be stored with last_8 in a single
                // 32-bit value.
                uint16_t rollovers;
            } as_values;
        };
        std::atomic<int32_t> g_last_time_and_rollovers { 0 };
        static_assert(sizeof(LastTimeAndRolloversState) <= sizeof(g_last_time_and_rollovers),
            "LastTimeAndRolloversState does not fit in a single atomic word");

        // We use timeGetTime() to implement TimeTicks::Now().  This can be problematic
        // because it returns the number of milliseconds since Windows has started,
        // which will roll over the 32-bit value every ~49 days.  We try to track
        // rollover ourselves, which works if TimeTicks::Now() is called at least every
        // 48.8 days (not 49 days because only changes in the top 8 bits get noticed).
        TimeTicks RolloverProtectedNow()
        {
            LastTimeAndRolloversState state;
            DWORD now; // DWORD is always unsigned 32 bits.

            // Fetch the "now" and "last" tick values, updating "last" with "now" and
            // incrementing the "rollovers" counter if the tick-value has wrapped back
            // around. Atomic operations ensure that both "last" and "rollovers" are
            // always updated together.
            int32_t original = g_last_time_and_rollovers.load(std::memory_order_acquire);
            while (true) {
                state.as_opaque_32 = original;
                now = g_tick_function();
                uint8_t now_8 = static_cast<uint8_t>(now >> 24);
                if (now_8 < state.as_values.last_8)
                    ++state.as_values.rollovers;
                state.as_values.last_8 = now_8;

                // If the state hasn't changed, exit the loop.
                if (state.as_opaque_32 == original)
                    break;

                // Save the changed state. If the existing value is unchanged from the
                // original, exit the loop.
                if (g_last_time_and_rollovers.compare_exchange_weak(
                        original, state.as_opaque_32, std::memory_order_acq_rel)) {
                    break;
                }

                // Another thread has done something in between so retry from the top.
                // {original} has been updated by the {compare_exchange_weak}.
            }

            return TimeTicks() + TimeDelta::FromMilliseconds(now + (static_cast<uint64_t>(state.as_values.rollovers) << 32));
        }

        // Discussion of tick counter options on Windows:
        //
        // (1) CPU cycle counter. (Retrieved via RDTSC)
        // The CPU counter provides the highest resolution time stamp and is the least
        // expensive to retrieve. However, on older CPUs, two issues can affect its
        // reliability: First it is maintained per processor and not synchronized
        // between processors. Also, the counters will change frequency due to thermal
        // and power changes, and stop in some states.
        //
        // (2) QueryPerformanceCounter (QPC). The QPC counter provides a high-
        // resolution (<1 microsecond) time stamp. On most hardware running today, it
        // auto-detects and uses the constant-rate RDTSC counter to provide extremely
        // efficient and reliable time stamps.
        //
        // On older CPUs where RDTSC is unreliable, it falls back to using more
        // expensive (20X to 40X more costly) alternate clocks, such as HPET or the ACPI
        // PM timer, and can involve system calls; and all this is up to the HAL (with
        // some help from ACPI). According to
        // http://blogs.msdn.com/oldnewthing/archive/2005/09/02/459952.aspx, in the
        // worst case, it gets the counter from the rollover interrupt on the
        // programmable interrupt timer. In best cases, the HAL may conclude that the
        // RDTSC counter runs at a constant frequency, then it uses that instead. On
        // multiprocessor machines, it will try to verify the values returned from
        // RDTSC on each processor are consistent with each other, and apply a handful
        // of workarounds for known buggy hardware. In other words, QPC is supposed to
        // give consistent results on a multiprocessor computer, but for older CPUs it
        // can be unreliable due bugs in BIOS or HAL.
        //
        // (3) System time. The system time provides a low-resolution (from ~1 to ~15.6
        // milliseconds) time stamp but is comparatively less expensive to retrieve and
        // more reliable. Time::EnableHighResolutionTimer() and
        // Time::ActivateHighResolutionTimer() can be called to alter the resolution of
        // this timer; and also other Windows applications can alter it, affecting this
        // one.

        TimeTicks InitialTimeTicksNowFunction();

        // See "threading notes" in InitializeNowFunctionPointer() for details on how
        // concurrent reads/writes to these globals has been made safe.
        using TimeTicksNowFunction = decltype(&TimeTicks::Now);
        TimeTicksNowFunction g_time_ticks_now_function = &InitialTimeTicksNowFunction;
        int64_t g_qpc_ticks_per_second = 0;

// As of January 2015, use of <atomic> is forbidden in Chromium code. This is
// what std::atomic_thread_fence does on Windows on all Intel architectures when
// the memory_order argument is anything but std::memory_order_seq_cst:
#define ATOMIC_THREAD_FENCE(memory_order) _ReadWriteBarrier();

        TimeDelta QPCValueToTimeDelta(LONGLONG qpc_value)
        {
            // Ensure that the assignment to |g_qpc_ticks_per_second|, made in
            // InitializeNowFunctionPointer(), has happened by this point.
            ATOMIC_THREAD_FENCE(memory_order_acquire);

            DCHECK_GT(g_qpc_ticks_per_second, 0);

            // If the QPC Value is below the overflow threshold, we proceed with
            // simple multiply and divide.
            if (qpc_value < TimeTicks::kQPCOverflowThreshold) {
                return TimeDelta::FromMicroseconds(
                    qpc_value * TimeTicks::kMicrosecondsPerSecond / g_qpc_ticks_per_second);
            }
            // Otherwise, calculate microseconds in a round about manner to avoid
            // overflow and precision issues.
            int64_t whole_seconds = qpc_value / g_qpc_ticks_per_second;
            int64_t leftover_ticks = qpc_value - (whole_seconds * g_qpc_ticks_per_second);
            return TimeDelta::FromMicroseconds(
                (whole_seconds * TimeTicks::kMicrosecondsPerSecond) + ((leftover_ticks * TimeTicks::kMicrosecondsPerSecond) / g_qpc_ticks_per_second));
        }

        TimeTicks QPCNow() { return TimeTicks() + QPCValueToTimeDelta(QPCNowRaw()); }

        bool IsBuggyAthlon(const CPU& cpu)
        {
            // On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is unreliable.
            return strcmp(cpu.vendor(), "AuthenticAMD") == 0 && cpu.family() == 15;
        }

        void InitializeTimeTicksNowFunctionPointer()
        {
            LARGE_INTEGER ticks_per_sec = {};
            if (!QueryPerformanceFrequency(&ticks_per_sec))
                ticks_per_sec.QuadPart = 0;

            // If Windows cannot provide a QPC implementation, TimeTicks::Now() must use
            // the low-resolution clock.
            //
            // If the QPC implementation is expensive and/or unreliable, TimeTicks::Now()
            // will still use the low-resolution clock. A CPU lacking a non-stop time
            // counter will cause Windows to provide an alternate QPC implementation that
            // works, but is expensive to use. Certain Athlon CPUs are known to make the
            // QPC implementation unreliable.
            //
            // Otherwise, Now uses the high-resolution QPC clock. As of 21 August 2015,
            // ~72% of users fall within this category.
            TimeTicksNowFunction now_function;
            CPU cpu;
            if (ticks_per_sec.QuadPart <= 0 || !cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
                now_function = &RolloverProtectedNow;
            } else {
                now_function = &QPCNow;
            }

            // Threading note 1: In an unlikely race condition, it's possible for two or
            // more threads to enter InitializeNowFunctionPointer() in parallel. This is
            // not a problem since all threads should end up writing out the same values
            // to the global variables.
            //
            // Threading note 2: A release fence is placed here to ensure, from the
            // perspective of other threads using the function pointers, that the
            // assignment to |g_qpc_ticks_per_second| happens before the function pointers
            // are changed.
            g_qpc_ticks_per_second = ticks_per_sec.QuadPart;
            ATOMIC_THREAD_FENCE(memory_order_release);
            g_time_ticks_now_function = now_function;
        }

        TimeTicks InitialTimeTicksNowFunction()
        {
            InitializeTimeTicksNowFunctionPointer();
            return g_time_ticks_now_function();
        }

#undef ATOMIC_THREAD_FENCE

    } // namespace

    // static
    TimeTicks TimeTicks::Now()
    {
        // Make sure we never return 0 here.
        TimeTicks ticks(g_time_ticks_now_function());
        DCHECK(!ticks.IsNull());
        return ticks;
    }

    // static
    bool TimeTicks::IsHighResolution()
    {
        if (g_time_ticks_now_function == &InitialTimeTicksNowFunction)
            InitializeTimeTicksNowFunctionPointer();
        return g_time_ticks_now_function == &QPCNow;
    }

#else // V8_OS_WIN

    TimeTicks TimeTicks::Now()
    {
        int64_t ticks;
#if V8_OS_MACOSX
        static struct mach_timebase_info info;
        if (info.denom == 0) {
            kern_return_t result = mach_timebase_info(&info);
            DCHECK_EQ(KERN_SUCCESS, result);
            USE(result);
        }
        ticks = (mach_absolute_time() / Time::kNanosecondsPerMicrosecond * info.numer / info.denom);
#elif V8_OS_SOLARIS
        ticks = (gethrtime() / Time::kNanosecondsPerMicrosecond);
#elif V8_OS_POSIX
        ticks = ClockNow(CLOCK_MONOTONIC);
#else
#error platform does not implement TimeTicks::HighResolutionNow.
#endif // V8_OS_MACOSX
        // Make sure we never return 0 here.
        return TimeTicks(ticks + 1);
    }

    // static
    bool TimeTicks::IsHighResolution()
    {
#if V8_OS_MACOSX
        return true;
#elif V8_OS_POSIX
        static bool is_high_resolution = IsHighResolutionTimer(CLOCK_MONOTONIC);
        return is_high_resolution;
#else
        return true;
#endif
    }

#endif // V8_OS_WIN

    bool ThreadTicks::IsSupported()
    {
#if (defined(_POSIX_THREAD_CPUTIME) && (_POSIX_THREAD_CPUTIME >= 0)) || defined(V8_OS_MACOSX) || defined(V8_OS_ANDROID) || defined(V8_OS_SOLARIS)
        return true;
#elif defined(V8_OS_WIN)
        return IsSupportedWin();
#else
        return false;
#endif
    }

    ThreadTicks ThreadTicks::Now()
    {
#if V8_OS_MACOSX
        return ThreadTicks(ComputeThreadTicks());
#elif (defined(_POSIX_THREAD_CPUTIME) && (_POSIX_THREAD_CPUTIME >= 0)) || defined(V8_OS_ANDROID)
        return ThreadTicks(ClockNow(CLOCK_THREAD_CPUTIME_ID));
#elif V8_OS_SOLARIS
        return ThreadTicks(gethrvtime() / Time::kNanosecondsPerMicrosecond);
#elif V8_OS_WIN
        return ThreadTicks::GetForThread(::GetCurrentThread());
#else
        UNREACHABLE();
#endif
    }

#if V8_OS_WIN

    typedef BOOL(__stdcall* PFN_QueryThreadCycleTime)(HANDLE ThreadHandle,
        PULONG64 CycleTime);
    static PFN_QueryThreadCycleTime s_QueryThreadCycleTime = NULL;

    inline BOOL QueryThreadCycleTimeIsLoaded()
    {
        static BOOL s_is_init = FALSE;
        if (!s_is_init) {
            HMODULE handle = GetModuleHandle(L"Kernel32.dll");
            s_QueryThreadCycleTime = (PFN_QueryThreadCycleTime)GetProcAddress(
                handle, "QueryThreadCycleTime");
            s_is_init = TRUE;
        }
        return NULL != s_QueryThreadCycleTime;
    }

    inline BOOL QueryThreadCycleTimeXp(HANDLE ThreadHandle, PULONG64 CycleTime)
    {
        if (QueryThreadCycleTimeIsLoaded())
            return s_QueryThreadCycleTime(ThreadHandle, CycleTime);

        return FALSE;
    }

    ThreadTicks ThreadTicks::GetForThread(const HANDLE& thread_handle)
    {
        DCHECK(IsSupported());

        // Get the number of TSC ticks used by the current thread.
        ULONG64 thread_cycle_time = 0;
        QueryThreadCycleTimeXp(thread_handle, &thread_cycle_time); // SUPPORT_XP_CODE

        // Get the frequency of the TSC.
        double tsc_ticks_per_second = TSCTicksPerSecond();
        if (tsc_ticks_per_second == 0)
            return ThreadTicks();

        // Return the CPU time of the current thread.
        double thread_time_seconds = thread_cycle_time / tsc_ticks_per_second;
        return ThreadTicks(
            static_cast<int64_t>(thread_time_seconds * Time::kMicrosecondsPerSecond));
    }

    // static
    bool ThreadTicks::IsSupportedWin()
    {
        static bool is_supported = base::CPU().has_non_stop_time_stamp_counter() && !IsQPCReliable();
#if 1 // def SUPPORT_XP_CODE
        static bool is_to_find = false;
        if (!is_to_find) {
            HMODULE module = GetModuleHandle(L"Kernel32.dll");
            FARPROC func = GetProcAddress(module, "QueryThreadCycleTime");
            is_supported &= !!func;
            is_to_find = true;
        }
#endif
        return is_supported;
    }

    // static
    void ThreadTicks::WaitUntilInitializedWin()
    {
        while (TSCTicksPerSecond() == 0)
            ::Sleep(10);
    }

#ifdef V8_HOST_ARCH_ARM64
#define ReadCycleCounter() _ReadStatusReg(ARM64_PMCCNTR_EL0)
#else
#define ReadCycleCounter() __rdtsc()
#endif

    double ThreadTicks::TSCTicksPerSecond()
    {
        DCHECK(IsSupported());

        // The value returned by QueryPerformanceFrequency() cannot be used as the TSC
        // frequency, because there is no guarantee that the TSC frequency is equal to
        // the performance counter frequency.

        // The TSC frequency is cached in a static variable because it takes some time
        // to compute it.
        static double tsc_ticks_per_second = 0;
        if (tsc_ticks_per_second != 0)
            return tsc_ticks_per_second;

        // Increase the thread priority to reduces the chances of having a context
        // switch during a reading of the TSC and the performance counter.
        int previous_priority = ::GetThreadPriority(::GetCurrentThread());
        ::SetThreadPriority(::GetCurrentThread(), THREAD_PRIORITY_HIGHEST);

        // The first time that this function is called, make an initial reading of the
        // TSC and the performance counter.
        static const uint64_t tsc_initial = ReadCycleCounter();
        static const uint64_t perf_counter_initial = QPCNowRaw();

        // Make a another reading of the TSC and the performance counter every time
        // that this function is called.
        uint64_t tsc_now = ReadCycleCounter();
        uint64_t perf_counter_now = QPCNowRaw();

        // Reset the thread priority.
        ::SetThreadPriority(::GetCurrentThread(), previous_priority);

        // Make sure that at least 50 ms elapsed between the 2 readings. The first
        // time that this function is called, we don't expect this to be the case.
        // Note: The longer the elapsed time between the 2 readings is, the more
        //   accurate the computed TSC frequency will be. The 50 ms value was
        //   chosen because local benchmarks show that it allows us to get a
        //   stddev of less than 1 tick/us between multiple runs.
        // Note: According to the MSDN documentation for QueryPerformanceFrequency(),
        //   this will never fail on systems that run XP or later.
        //   https://msdn.microsoft.com/library/windows/desktop/ms644905.aspx
        LARGE_INTEGER perf_counter_frequency = {};
        ::QueryPerformanceFrequency(&perf_counter_frequency);
        DCHECK_GE(perf_counter_now, perf_counter_initial);
        uint64_t perf_counter_ticks = perf_counter_now - perf_counter_initial;
        double elapsed_time_seconds = perf_counter_ticks / static_cast<double>(perf_counter_frequency.QuadPart);

        const double kMinimumEvaluationPeriodSeconds = 0.05;
        if (elapsed_time_seconds < kMinimumEvaluationPeriodSeconds)
            return 0;

        // Compute the frequency of the TSC.
        DCHECK_GE(tsc_now, tsc_initial);
        uint64_t tsc_ticks = tsc_now - tsc_initial;
        tsc_ticks_per_second = tsc_ticks / elapsed_time_seconds;

        return tsc_ticks_per_second;
    }
#undef ReadCycleCounter
#endif // V8_OS_WIN

} // namespace base
} // namespace v8
